A conventional line monitor comprises two optical fibers disposed opposite to each other, lenses for optically coupling the optical fibers to each other, a reflecting mirror disposed between the lenses, a lens for receiving light reflected from the reflecting mirror, and a photodetector for receiving light from the lens.
Light output from one optical fiber is brought to parallel beams by means of the lens, and a part thereof is vertically reflected by means of the reflecting mirror disposed at an angle of 45.degree. to the direction of advance with the remaining component being passed through the mirror. The transmitted light is focused by means of the lens and coupled to the other optical fiber.
On the other hand, the reflected light is focused by means of the lens and coupled to the photodetector.
In the conventional optical line monitor, however, there is a disadvantage in that, since the branching film does not have the wavelength selectivity, when an optical signal containing a noise light with a wavelength different from that of the contemplated optical signal enters the line monitor, the noise light is also branched, often making it impossible to precisely monitor only the contemplated optical signal. An additional disadvantage of the conventional optical line monitor is such that when a plurality of optical signals in a wavelength multiplexed state are input, the optical signals with different wavelengths cannot be monitored separately from each other. Further, since the number of optical parts constituting the line monitor is large, it is difficult to reduce the size of the line monitor. Furthermore, the coupling loss is disadvantageously large because the optical signal goes out from an optical fiber into air and then again is introduced into the optical fiber.